Polylactate (PLA) is a typical biodegradable polymer originated from lactate, which has a variety of applications as a common or a medical polymer. At present, PLA is being prepared by polymerizing lactate which is produced by fermenting microorganisms, but only low molecular weight PLA (1000-5000 dalton) is produced by direct polymerization of lactate. To synthesize high molecular weight (>100,000 dalton) of PLA, a method polymerizing low molecular weight PLA obtained by direct polymerization of lactate with a chain coupling agent can be used. However, it has disadvantages like that the process for preparing PLA of high molecular weight is complicated due to the addition of a solvent or a chain coupling agent, and also it isn't easy to remove them. At present, in the process for preparing commercially available PLA of high molecular weight, a method, in which lactate is converted into lactide to synthesize PLA by cyclodehydration of the lactide ring, is being used.
Meanwhile, polyhydroxyalkanoate (PHA) is a polyester which microorganisms accumulate therein as a carbon and energy storage compound when other nutritive elements, for example, phosphorus, nitrogen, magnesium, oxygen, are deficient while the carbon source is in excess. PHA is recognized as an alternative material for synthesized plastics since it has similar properties to synthetic polymers originating from petroleum, and, at the same time, shows an excellent biodegradation property.
The existing PHA is divided into SCL-PHA (short-chain-length PHA) having short carbon chains and MCL-PHA(medium-chain-length PHA) having long carbon chains. A gene synthesizing PHA was cloned from Ralstonia eutropha, Pseudomonas sp. Microorganism, and PHA consisting of various monomers was synthesized by recombinant microorganisms (Qi et al., FEMS Microbiol. Lett., 157:155, 1997; Qi et al., FEMS Microbiol. Lett., 167:89, 1998; Langenbach et al., FEMS Microbiol. Lett., 150:303, 1997; WO 01/55436; U.S. Pat. No. 6,143,952; WO 98/54329; and WO 99/61624).
To produce PHA in microorganisms, an enzyme which converts microorganisms' metabolites into a PHA monomer and PHA synthase which synthesizes a PHA polymer using the PHA monomers are required. PHA synthase synthesizes PHA using hydroxyacyl-CoA as a substrate and alpha-ketothiolase (PhaA), acetoacetyl-CoA reductase (PhaB), cloned from Ralstonia eutropha etc., 3-hydroxydecanoyl-ACP:CoA transferase (PhaG) cloned from Pseudomonas sp., (R)-specific enoyl-CoA hydratase (PhaJ) derived from Aeromonas caviae and Pseudomonas aeruginosa (Fukui et al., J. Bacteriol., 180:667, 1998; Tsage et al., FEMS Microbiol. Lett., 184:193, 2000), 3-ketoacyl-ACP reductase (FabG) derived from E. coli, Pseudomonas aeruginosa, etc. (Taguchi et al., FEMS Microbiol. Lett., 176:183, 1999; Ren et al., J. Bacteriol., 182:2978, 2000; Park et al., FEMS Microbiol. Lett., 214:217, 2002), phosphotransbutylase (Ptb) and butyrate kinase (Buk) derived from Clostridium acetobutyricum (Liu and Steinbuchel, Appl Environ Microbiol, 66:739, 2000), Cat2 derived from Clostridium kluyveri (Hein et al. FEMS Microbiol. Lett., 15:411, 1997), etc. are known as enzymes capable of generating hydroxyacyl-CoA which is a substrate of PHA.
Various kinds of PHAs have been synthesized with these enzymes using hydroxyalkanoates hydroxylated at various positions in the carbon chain (mainly the 3, 4, 5, and 6 positions).
However, it has been reported that it has little PHA synthase activity on hydroxyalkanoate which is hydroxylated at the 2-position (Zhang et al., Appl. Microbiol. Biotechnol., 56:131, 2001; Valentin and Steinbuchel, Appl. Microbiol. Biotechnol., 40:699, 1994). Thus far, there have been reports of PHA synthase activity on lactyl-CoA measured in vitro, but PHA synthase activity on lactyl-CoA is very weak (Zhang et al., Appl. Microbiol. Biotechnol., 56:131, 2001; Valentin and Steinbuchel, Appl. Microbiol. Biotechnol., 40:699, 1994). That is, there are no examples of natural production or production by recombinant cells of PHA and its copolymers because a hydroalkanoate, such as lactate hydroxylated at the 2-carbon position, is not a suitable substrate for PHA synthase.